Generally, a particle beam therapy system includes a beam generation apparatus that generates a charged particle beam; an accelerator that is connected to the beam generation apparatus and accelerates the generated charged particle beam; a charged particle-beam transport system that transports the charged particle beam emitted after accelerated up to a specified energy by the accelerator; and a particle beam irradiation apparatus that is placed at the downstream side of the beam transport system, for radiating the charged particle beam to an irradiation target.
As to the beam transport system, generally, its optical parameters are designed while setting two reference points at one point on the beam trajectory of the accelerator and at a beam irradiation position (isocenter). The one point on the beam trajectory of the accelerator is given as a start point in the optical parameter design, and the beam irradiation position, in particular, the isocenter which is the center of that position, is given as a terminal point in the optical parameter design. Specifically, the beam transport system transports the beam so that the beam reaches the irradiation position, in such a manner that the intensity of each electromagnet in said beam transport system is calculated using, as a design value, a beam initial condition at a communication point (start point) between the accelerator and the beam transport system (High Energy Beam Transport System (HEBT System)), followed by setting that intensity (excitation current) to the electromagnet.
When the accelerator is a small-size medical synchrotron, because of an unexpected magnetic field by the electromagnets of the synchrotron (a magnetic field generated due to machining error, etc.), there are cases where, even if the initial value of the synchrotron is an ideal value (design value), the Hardt condition is not satisfied even when a six-pole electromagnet is properly arranged, so that a separatrix branch for emission differs depending on the energy. Thus, there is a phenomenon (chromatic aberration) in which, an angle (inclination) or position of the beam at the communication point differs depending on an amount of the energy, so that, though depending on a beam emission method of the accelerator, a phenomenon occurs in which the position of the beam moves or the diameter of the beam increases, at the irradiation position.
The reason of occurrence of the chromatic aberration, that is the phenomenon in which a difference emerges in the angle (inclination) or position of the beam, will be described. FIG. 15 is a graph illustrating a movement of the beam in a phase space at the start point, and FIG. 16 is diagrams each illustrating beam trajectories. In FIG. 15, the abscissa represents a distance ΔX in an x-direction perpendicular to the central axis of the beam trajectories, and the ordinate represents an inclination ΔX′ at ΔX relative to the central axis of the beam trajectories. In FIG. 16, the abscissa represents an s-axis extending in a beam traveling direction, and the ordinate represents the distance ΔX in the x-direction. In FIG. 16, there are shown positions of bending electromagnets 63 and quadrupole electromagnets 64 each related to a change in beam trajectory, and a start point S and a terminal point T. Shown at the upper side in FIG. 16 is in an ideal case of beam emission, and shown at the lower side in FIG. 16 is in a case where the beam emission is deviated from the ideal one.
Heretofore, a beam optical system in the beam transport system has been designed on the assumption that there is no movement of the beam at the start point S, and on the presumption that the beam has an assumed-phase spatial distribution shown as an ellipse in FIG. 15. However, an actual beam has phase spatial distributions 60 due to temporal change in phase spatial distribution as shown at 61a, 61b and 61c. As to the actual beam, its current value intermittently goes back and forth between zero and a value other than zero as shown in FIG. 5, so that the phase spatial distribution of the beam differs among at Times t1, t2 and t3. For example, a phase spatial distribution of the beam is: the phase spatial distribution 61a at Time t1 (at the beginning of the spill); the phase spatial distribution 61b at Time t2 (at the middle of the spill); and the phase spatial distribution 61c at Time t3 (at the end of the spill).
In the ideal case of beam emission, as shown at the upper side in FIG. 16, even with no movement of the beam in the phase space at the start point S, variations occur at the upstream side as indicated by beam trajectories 65a, 65b and 65c; nevertheless, in the downstream side, adjustment of the excitation currents of the bending electromagnets 63 and the quadrupole electromagnets 64 makes it possible to adjust the beam trajectories to be matched to the beam axis (s-axis) so that no chromatic aberration occurs at the terminal point T. However, in the case where the beam emission is deviated from such an ideal state, namely, in the case where the phase spatial distribution varies temporally, as shown at the lower side in FIG. 16, even at the downstream side, variations occur as indicated by beam trajectories 66a, 66b and 66c, so that a chromatic aberration occurs at the terminal point T as an irradiation position. For example, the beam trajectory 66a is a trajectory corresponding to the phase spatial distribution 61a, the beam trajectory 66b is a trajectory corresponding to the phase spatial distribution 61b, and the beam trajectory 66c is a trajectory corresponding to the phase spatial distribution 61c. In the case where the beam emission is deviated from the ideal state, because of the occurrence of the chromatic aberration at the terminal point T, the beam diameter becomes enlarged and the beam position (gravity-center position) becomes placed apart from the beam axis (s-axis).
In an actual beam transport system, there is a temporal variation in the phase spatial distribution of the beam at the start point S, so that if no consideration is paid to the temporal variation in the phase spatial distribution of the beam, a chromatic aberration occurs at the terminal point T as described above. Thus, in order to nullify the chromatic aberration at the terminal point T, it is necessary to take into consideration the temporal variation in the phase spatial distribution of the beam at the start point S.
In Patent Document 1, there is described a method for accomplishing automated adjustment of the beam size in order to make adjustment of the beam size easier. The charged-particle beam transport apparatus of Patent Document 1 includes: a sensitivity calculator that calculates a sensitivity matrix indicative of a relationship of a beam size relative to a converging force of a beam focusing device such as a quadrupole electromagnet, etc., on the basis of the beam sizes and beam profiles measured by a plurality of profile monitors placed between the outlet of the accelerator and the inlet of the irradiation apparatus; and an excitation-current correction-amount calculator that calculates using the sensitivity matrix, a beam-converging force from a target value set for adjusting the beam size; so that the beam focusing device is controlled using the excitation current calculated by the excitation-current correction-amount calculation device. According to the adjustment method of the beam size in Patent Document 1, the sensitivity matrix is calculated from the beam sizes and the beam profiles measured by the profile monitors after rough adjustment for the beam transportation, the excitation current of each beam focusing device is calculated using the sensitivity matrix, and then an adjustment is performed by exciting said each beam focusing device using the excitation current. This method is repeated until the beam size becomes sufficiently near to an intended value.